The Peak in North American Ichneumonid Species Richness Lies Between 38 Degrees and 42 Degrees N

An evaluation of the geographic distribution of °60% of the 2809 described Nearctic Ichneumonidae shows that of the species whose latitudinal ranges contact a given latitudinal band, the number per million square kilometres is greatest between latitude 37.5° and 42.4°N, with °900 species/106 km2. The most severe absolute drop in species richness occurs in moving southward from the 37.5°—39.9° band to the 38.0°—37.4° band, with virtually no change in land mass over the same latitudinal change, and the smallest drop in species richness occurs in moving from the 32.5°—34.9° band to the 30.0°—32.4° band, where the land mass is almost halved. Of species whose southern range ends in a particular latitudinal band, the number per million square kilometres is greatest for 32.5°—34.4°N and drops away gradually in both directions. These results are consistent with the hypothesis that parasitic Hymenoptera do not increase in species richness with decreasing latitude to the same degree as their hosts. Various hypotheses are discussed as to why this might occur in Ichneumonidae.